Fixes#338
Add methods to update block type and labels to enable us to refactor HCL
configurations such as renaming Terraform resources.
- `*Block.SetType(typeName string)`
- `*Block.SetLabels(labels []string)`
Some additional notes about SetLabels:
Since we cannot assume that old and new labels are equal in length,
remove old labels and insert new ones before TokenOBrace.
To implement this, I also added the following methods.
- `*nodes.Insert(pos *node, c nodeContent) *node`
- `*nodes.InsertNode(pos *node, n *node) *node`
They are similar to the existing Append / AppendNode,
but insert a node before a given position.
